Please answer these: Q1. i. In what two ways combustion can be useful to us? Explain by giving examples. ii. Explain why decomposition of sugar on heating is an irreversible change. Q2. In order to answer this question you need to look around your immediate surroundings to find examples of rusting of iron or any other metal. i. What makes iron rust? ii. Why is the rusting of iron a problem? Give any 3 examples from daily life where rusting has been a disadvantage. iii. Suggest any two ways that can help prevent rusting of iron. Q3. i. Give two differences between a physical change and a chemical change. ii. Explain whether condensation of water vapour is a physical or a chemical change. iii. Write word equations for the following reactions: a. Breaking down/ electrolysis of water b. synthesis of water c. decomposition of sugar d. photosynthesis in plants

1) combustion can be used as a source of energy.

Combustion is used to power rockets

ii) Decomposition of sugar is a chemical change and new substances are formed.

2)corrosion of pipes

Corrosion of bridges

Corrosion of knives and blades at home

I) iron rusts due to the action of air and moisture bon the metal setting up an electrochemical process in which the iron is the anode. This leads to the corrosion of the iron metal.

ii) Rusting is a problem because it reduces the economic value of iron. Rust weakens iron and threatens the integrity of materials made from iron.

Iron rusting may lead to the collapse of a bridge

Iron rusting makes a blade blunt

Iron rusting weakens pipes made of iron

iii) Rusting of iron can be prevented by galvanizing the iron or by painting the iron.

3) chemical change leads to the formation of a new substance while physical change does not lead to the formation of a new substance.

Chemical changes are not easily reversible while physical changes are easily reversible.

ii)condensation of water vapour is a physical change

iii)

Electrolysis of water;

Two moles of water---> two moles of hydrogen gas + one mole of oxygen gas

Synthesis of water;

two moles of hydrogen gas + one mole of oxygen gas>Two moles of water

Decomposition of sugar;

One mole of Sucrose---> twelve moles of carbon + eleven moles of water + heat

Six moles of carbon dioxide + six moles of water +energy> one mole of sugar + six moles of oxygen

Explanation:

Combustion is useful as a source of energy because it is used in power generation. Currently, combustion is being used to power rockets. It is also used as a way of providing heat at home.

One of the characteristics of a chemical change is that it leads to the formation of new substances. Since the decomposition of sugar leads to the formation of carbon and water, it is an irreversible chemical change.

When water is heated, it is converted into vapour which can settle on a surface and convert back to liquid water. This is a physical change.

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